Paving of roadways with asphalt is generally carried out by an asphalt paving machine and a number of supply trucks or a material transfer vehicle which transport the asphalt from an asphalt production plant to the paving machine. The paving machine generally is self-propelled and driven by a wheeled or tracked drive system. In a common type of paving machine, a hopper is located at the front end of the machine to receive asphalt from a truck, and a conveyor system typically comprised of one or more slat conveyors transfers the asphalt from the hopper to the road bed or other surface to be paved, in one or more windrows. In another type of paving machine, a gravity-feed hopper is mounted so that the asphalt from the hopper is directed to the road bed or other surface to be paved. A transverse distributing auger is mounted near the rear of the machine to distribute the asphalt across the width of the roadway or lane to be paved and to level it. A floating screed located at the rear end of the machine behind the distributing auger compacts the asphalt and forms the asphalt mat.
It is frequently desirable to apply an asphalt emulsion, liquid asphalt or a similar substance (commonly referred to as “tack” or “tack material”) to the surface to be paved prior to distributing and compacting the asphalt into a mat to bind the asphalt to the underlying surface. Tack is typically applied just prior to a paving operation by being sprayed onto the surface to be paved from a spray bar extending transversely over the surface. A tack truck operated independently of the paving machine is usually employed for this purpose. The typical tack truck includes a self-propelled chassis on which are mounted a heated tack storage tank and a tack spray assembly. The truck travels in front of the paving machine while applying a layer of tack to the surface to be paved. The truck travels at 5-10 miles per hour, considerably faster than the 30-120 feet per minute operational speed of the paving machine. Consequently, in order to avoid applying an asphalt mat to a surface on which a layer of tack has prematurely cooled by the passage of time (which premature cooling degrades the binding performance of the tack), the truck must stop periodically in order to wait for the slower paving machine. Tack application systems which employ a tack truck are described in U.S. Pat. No. 4,793,731 of Gnesa, U.S. Pat. No. 4,828,429 of Kirchner et al. and U.S. Pat. No. 4,684,289 of Gnesa.
Among the disadvantages attending the use of a separate tack application vehicle is that the paving machine (and perhaps other vehicles) must travel across the surface to which tack has been applied before the asphalt mat is laid down. This disturbs the tack layer on the surface to be paved and transfers the sticky tack material to the wheels, treads or other components of the paving machine. Furthermore, if the tack truck applies a tack layer too far ahead of the paving machine, the tack material can cool before application of the asphalt mat, which degrades its binding characteristics. Finally, the tack truck requires a separate operator from the paving machine, and there are additional costs associated with maintaining and operating the truck.
Consequently, it is also known to modify a paving machine by installing a tack spray assembly in front of the distributing auger, thereby insuring that the applied tack layer is almost immediately covered by the asphalt mat. Paving machines which include such assemblies are described in U.S. Pat. No. 5,131,788 of Hulicsko and U.S. Pat. No. 5,851,085 of Campbell. However, most paving machines include a screed having extensions that permit adjustment of the width of the asphalt mat being applied. Some such extensions can increase the width to be paved from ten feet to twenty feet or more. In such paving machines which include a tack spray assembly, the width of the spray assembly must also be adjustable to permit the application of tack across the entire width to be paved. Generally this is accomplished by providing a pair of parallel spray bars across the width of the machine, which spray bars may be moved with respect to each other to provide tack coverage across the entire width to be paved. One problem with conventional paving machines which include a pair of moveable tack spray bars is that paving a width less than the maximum allowed by the fully-extended spray bars results in an uneven application of tack because of overlap of spray from adjacent nozzles on the spray bar. U.S. Pat. No. 5,354,148 of Reymonet addresses this problem by controlling the opening and closing of the various spray nozzles using a plurality of servo valves. A different solution to this problem is disclosed by Japanese Patent Publication No. 10072805 of Niigata Engineering Company, Ltd., which describes a spray assembly comprising a first spray bar, a second spray bar, and an excess spray collector pan that is affixed to the front of the first spray bar. This collector pan is arranged so as to intersect the spray from the nozzles of the second spray bar when the spray bars are moved so that the spray from one or more nozzles on the second spray bar overlaps the spray from one or more nozzles on the first spray bar. The disadvantage to this arrangement arises from the fact that all of the tack material that is dispensed through a nozzle providing overlapping spray must be collected in the collector pan and pumped back into the system.
It would be advantageous if a spray assembly for a paving machine could be devised that would permit the application of tack material evenly across various widths of paving without the disadvantages of prior systems. It would also be advantageous if such an assembly could permit the application of a consistent and even layer of tack material when the paving machine is operated at various paving speeds.